1. Field of the Invention
The present invention relates to an organic electroluminescent device and the fabrication method thereof.
2. Description of the Background Art
In recent years, with increasing diversity in information equipment, there is a growing need for flat panel display elements requiring smaller power consumption than generally used CRTs (Cathode Ray Tube). As one of the flat panel display elements, an organic electroluminescent (hereinafter abbreviated as an organic EL) element having such features as high efficiency, thin-type, light weight, and low dependence of angular-field-of-view is calling attention, with displays using these organic EL elements being under active development.
An organic EL element is a self light emitting element, in which electrons and holes are injected into a light emitting portion from an electron injection electrode and a hole injection electrode, respectively, and the injected electrons and holes are recombined at the light emitting center to bring an organic molecule into an excited state, so that the organic molecule emits fluorescent light when returning to a ground state from the excited state.
An emission color of the organic EL element can be varied by the choice of a fluorescent substance as a luminescent material, and therefore, the organic EL element is expected to find applications in such displays as multi-color and full-color displays. The organic EL element can also serve as a backlight for a liquid crystal display and the like because of the capability of surface-light emission at low voltage. Such organic EL elements are currently in the development stage for applications in small displays used in digital cameras, portable telephones, and the like.
The organic EL element is very sensitive to moisture: specifically, phenomena occur, such as the deterioration of the interface between a metal electrode and an organic layer affected by moisture, the stripping of electrodes, an increase in resistance of the metal electrode resulting from oxidation, or the deterioration of an organic material itself due to moisture. This causes increased drive voltage, the formation and growth of dark spots (non-luminescent defects), reduced luminance, or the like, resulting in the problem of the loss of sufficient reliability.
The organic EL element therefore cannot maintain sufficient reliability, unless the invasion of moisture thereto is prevented. For this reason, the structure as shown in FIG. 4 is used in order to prevent the invasion of moisture. FIG. 4 is a schematic cross-section of a conventional organic EL device.
In FIG. 4, a plurality of organic EL elements 50 are provided on a substrate 1. Each of the organic EL elements 50 includes, in sequence, a hole injection electrode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and an electron injection electrode. Only the hole injection electrode 2 is shown herein.
In the conventional organic EL device, a sealing agent 11 is applied to the outer periphery of the substrate 1, and a sealing can 20J made of a glass or a metal with a desiccant 31 inside is put above the substrate 1 so as to cover the plurality of organic EL elements 50. The sealing agent 11 is allowed to cure using ultraviolet light or heat, which bonds the sealing can 20J made of a metal or a glass to the substrate 1. The organic EL elements 50 are thus shielded from the outside-air.
In the organic EL device 900 of FIG. 4, the sealing can 20J is used for sealing off the organic EL elements 50. Here, the desiccant 31 is provided inside the sealing can 20J. In such a structure, the hole injection electrode 2, being in contact with the sealing agent 11, will undergo corrosion with an elapse of time due to the invasion of moisture into the sealing agent 11.
For this reason, a structure of an organic EL element has been proposed in which a photo-cured resin layer having moisture resistance is formed to cover the organic EL layers of the organic EL element, on top of which is fixed a small substrate having non-water permeability (refer to, for example JP 5-182759 A).
With this organic EL element structure, thinning of the organic EL element itself has been achieved, because the organic EL element is shielded from the outside-air using the moisture-resistant photo-cured resin layer and the non-water permeable substrate.
Also proposed is a structure of a display in which a lower electrode, a light emitting layer, and an upper electrode are formed in sequence on the substrate, on top of which is formed, in contact with the upper electrode, a sealing film made of a material having a refractive index less than 3.5 and higher than the refractive index of the atmosphere (refer to, for example, JP 2002-231443 A).
With this display structure, an improved efficiency of extracting the light emitted from the light emitting element through the upper electrode side has been achieved, because of reduced reflection of the emitted light on the surface of the upper electrode.
In the structures of the above-described patent documents 1 and 2, however, as the standing time for testing moisture resistance elapses, phenomena occur in the organic EL element and the display, such as decreased luminance, the formation of dark spots, or the expansion and advance thereof. As a result, the organic EL element undergoes further deterioration, which ultimately makes it impossible for use.